arXiv subjects

M. Takano

Publications and source records attributed to M. Takano.

At least 37 records · Page 2Linked to original sources

arXiv2007

A muon-spin relaxation study of BiMnO3

We present the results of muon-spin relaxation measurements on ferromagnetic BiMnO3. Below T_C=98.0(1) K oscillations in the time-dependence of the muon polarization are observed, characteristic of a quasistatic magnetic field at a single muon site, allowing us to probe the critical behaviour associated with the magnetic phase transition. We are able to suggest candidate muon sites on the basis of dipole field calculations. Close to T_C, fluctuations of the Mn^3+ moments are characteristic of critical behaviour while there is a sharp crossover to a region of fast dynamic fluctuations at higher temperatures.

cond-mat.str-el↗

arXiv2007

Variational Calculation for the Equation of State of Nuclear Matter at Finite Temperatures

An equation of state (EOS) for uniform nuclear matter is constructed at zero and finite temperatures with the variational method starting from the realistic nuclear Hamiltonian composed of the Argonne V18 and UIX potentials. The energy is evaluated in the two-body cluster approximation with the three-body-force contribution treated phenomenologically so as to reproduce the empirical saturation conditions. The obtained energies for symmetric nuclear matter and neutron matter at zero temperature are in fair agreement with those by Akmal, Pandharipande and Ravenhall, and the maximum mass of the neutron star is 2.2 Msolar. At finite temperatures, a variational method by Schmidt and Pandharipande is employed to evaluate the free energy, which is used to derive various thermodynamic quantities of nuclear matter necessary for supernova simulations. The result of this variational method at finite temperatures is found to be self-consistent.

nucl-th↗

arXiv2007

Two-Staged Magnetoresistance Driven by Ising-like Spin Sublattice in SrCo6O11

A two-staged, uniaxial magnetoresistive effect has been discovered in SrCo6O11 having a layered hexagonal structure. Conduction electrons and localized Ising spins are in different sublattices but their interpenetration makes the conduction electrons sensitively pick up the stepwise field-dependence of magnetization. The stepwise field-dependence suggests two competitive interlayer interactions between ferromagnetic Ising-spin layers, i.e., a ferromagnetic nearest-layer interaction and an antiferromagnetic next-nearest-layer interaction. This oxide offers a unique opportunity to study nontrivial interplay between conduction electrons and Ising spins, the coupling of which can be finely controlled by a magnetic field of a few Tesla.

cond-mat.str-el↗

arXiv2007

An Intrinsic Bond-Centered Electronic Glass with Unidirectional Domains in Underdoped Cuprates

Removing electrons from the CuO2 plane of cuprates alters the electronic correlations sufficiently to produce high-temperature superconductivity. Associated with these changes are spectral weight transfers from the high energy states of the insulator to low energies. In theory, these should be detectable as an imbalance between the tunneling rate for electron injection and extraction - a tunneling asymmetry. We introduce atomic-resolution tunneling-asymmetry imaging, finding virtually identical phenomena in two lightly hole-doped cuprates: Ca1.88Na0.12CuO2Cl2 and Bi2Sr2Dy0.2Ca0.8Cu2O8+d. Intense spatial variations in tunneling asymmetry occur primarily at the planar oxygen sites; their spatial arrangement forms a Cu-O-Cu bond centered electronic pattern without long range order but with 4a0-wide unidirectional electronic domains dispersed throughout (a0: the Cu-O-Cu distance). The emerging picture is then of a partial hole-localization within an intrinsic electronic glass evolving, at higher hole-densities, into complete delocalization and highest temperature superconductivity.

cond-mat.supr-con↗

arXiv2007

Angle-resolved photoemission studies of lattice polaron formation in the cuprate Ca2CuO2Cl2

To elucidate the nature of the single-particle excitations in the undoped parent cuprates, we have performed a detailed study of Ca2CuO2Cl2 using photoemission spectroscopy. The photoemission lineshapes of the lower Hubbard band are found to be well-described by a polaron model. By comparing the lineshape and temperature dependence of the lower Hubbard band with additional O 2p and Ca 3p states, we conclude that the dominant broadening mechanism arises from the interaction between the photohole and the lattice. The strength of this interaction was observed to be strongly anisotropic and may have important implications for the momentum dependence of the first doped hole states.

cond-mat.str-el↗

arXiv2006

Effects of annealing time on structural and magnetic properties of L10-FePt nanoparticles synthesized by the SiO2-nanoreactor method

We investigated effects of annealing time on structural and magnetic properties of the L10-FePt nanoparticles synthesized by the SiO2-nanoreacter method. The magnetization and powder X-ray diffraction studies revealed that the annealing at 900 oC for 9 hr could convert all of the fcc-nanoparticles to the well-crystallized L10 structure with a large coercivity while keeping their particle size. Such monodisperse and highly crystalline L10-FePt nanoparticles are a promising material for the realization of ultra-high density recording.

cond-mat.mtrl-sci↗

arXiv2006

Room-temperature ferromagnetism in Sr_(1-x)Y_xCoO_(3-delta) (0.2 < x < 0.25)

We have measured magnetic susceptibility and resistivity of Sr$_{1-x}$Y$_x$CoO$_{3-δ}$ ($x=$ 0.1, 0.15, 0.2, 0.215, 0.225, 0.25, 0.3, and 0.4), and have found that Sr$_{1-x}$Y$_x$CoO$_{3-δ}$ is a room temperature ferromagnet with a Curie temperature of 335 K in a narrow compositional range of 0.2 $\leq x\leq$ 0.25. This is the highest transition temperature among perovskite Co oxides. The saturation magnetization for $x=$ 0.225 is 0.25 $μ_B$/Co at 10 K, which implies that the observed ferromagnetism is a bulk effect. We attribute this ferromagnetism to a peculiar Sr/Y ordering.

cond-mat.str-el↗

arXiv2005

Design of a Polarised Positron Source Based on Laser Compton Scattering

We describe a scheme for producing polarised positrons at the ILC from polarised X-rays created by Compton scattering of a few-GeV electron beam off a CO2 or YAG laser. This scheme is very energy effective using high finesse laser cavities in conjunction with an electron storage ring.

physics.acc-ph↗

arXiv2005

Valence changes associated with the metal-insulator transition in Bi$_{1-x}$La$_x$NiO$_3$

Perovskite-type BiNiO$_3$ is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic. We have measured the photoemission and x-ray absorption (XAS) spectra of Bi$_{1-x}$La$_{x}$NiO$_{3}$ to investigate how the electronic structure changes with La doping. From Ni $2p$ XAS, we observed an increase of the valence of Ni from 2+ toward 3+. Combined with the core-level photoemission study, it was found that the average valence of Bi remains $\sim 4+$ and that the Ni valence behaves as $\sim (2+x)+$, that is, La substitution results in hole doping at the Ni sites. In the valence-band photoemission spectra, we observed a Fermi cutoff for $x>0$, consistent with the metallic behavior of the La-doped compounds. The Ni $2p$ XAS, Ni $2p$ core-level photoemission, and valence-band photoemission spectra were analyzed by configuration-interaction cluster-model calculation, and the spectral line shapes were found to be consistent with the gradual Ni$^{2+} \to$ Ni$^{3+}$ valence change.

cond-mat.str-el↗

arXiv2005

Pressure/temperature/substitution-induced melting of A-site charge disproportionation in Bi_(1-x)La_(x)NiO_3 (0 =< x =< 0.5)

Metal-insulator transitions strongly coupled with lattice were found in Bi1-xLaxNiO3. Synchrotron X-ray powder diffraction revealed that pressure (P ~ 3 GPa, T = 300 K), temperature (T ~ 340 K, x = 0.05), and La-substitution (x ~ 0.075, T = 300 K) caused the similar structural change from a triclinic (insulating) to an orthorhombic (metallic) symmetry, suggesting melting of the A-site charge disproportionation. Comparing crystal structure and physical properties with the other ANiO3 series, an electronic state of the metallic phase can be described as [A3+Ld, Ni2+L1-d], where a ligand-hole L contributes to a conductivity. We depicted a schematic P-T phase diagram of BiNiO3 including a critical point (3 GPa, 300 K) and an inhomogeneous region, which implies universality of ligand-hole dynamics in ANiO3 (A = Bi, Pr, Nd,...).

cond-mat.str-el↗

arXiv2004

Magnetic Phase Diagram of the Hole-doped Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ Cuprate Superconductor

We report on the magnetic phase diagram of a hole-doped cuprate Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$, which is free from buckling of CuO$_2$ planes, determined by muon spin rotation and relaxation. It is characterized by a quasi-static spin glass-like phase over a range of sodium concentration ($0.05\leq x\leq 0.12$), which is held between long range antiferromagnetic (AF) phase ($x\leq 0.02$) and superconducting phase where the system is non-magnetic for $x\geq 0.15$. The obtained phase diagram qualitatively agrees well with that commonly found for hole-doped high-\tc cuprates, strongly suggesting that the incomplete suppression of the AF order for $x>0.02$ is an essential feature of the hole-doped cuprates.

cond-mat.str-el↗

arXiv2004

Momentum anisotropy of the scattering rate in cuprate superconductors

We examine the momentum and energy dependence of the scattering rate of the high temperature cuprate superconductors using angle resolved photoemission spectroscopy. The scattering rate is of the form a + bw around the Fermi surface for under and optimal doping. The inelastic coefficient "b" is found to be isotropic. The elastic term, "a", however, is found to be highly anisotropic for under and optimally doped samples, with an anisotropy which correlates with that of the pseudogap. This is contrasted with heavily overdoped samples, which show an isotropic scattering rate and an absence of the pseudogap above T_c. We find this to be a generic property for both single and double layer compounds.

cond-mat.str-el↗

arXiv2004

Phase Transitions in Nucleonic Matter and Neutron-Star Cooling

A new scenario for neutron-star cooling is proposed, based on the correspondence between pion condensation, occurring in neutron matter due to critical spin-isospin fluctuations, and the metal-insulator phase transition in a two-dimensional electron gas. Beyond the threshold density for pion condensation, where neutron-star matter loses its spatial homogeneity, the neutron single-particle spectrum acquires an insulating gap that quenches neutron contributions to neutrino-production reactions and to the star's specific heat. In the liquid phase at densities below the transition point, spin-isospin fluctuations are found to play dual roles. On the one hand, they lead to a multi-sheeted neutron Fermi surface that extends to low momenta, thereby activating the normally forbidden direct-Urca cooling mechanism; on the other, they amplify the nodeless $P$-wave neutron superfluid gap while suppressing $S$-wave pairing. In this picture, lighter stars without a pion-condensed core experience slow cooling, while enhanced cooling occurs in heavier stars through direct-Urca emission from a narrow shell of the interior.

astro-ph↗

arXiv2004

A `checkerboard' electronic crystal state in lightly hole-doped Ca_{2-x}Na_{x}CuO_{2}Cl_{2}

The phase diagram of hole-doped copper oxides shows four different electronic phases existing at zero temperature. Familiar among these are the Mott insulator, high-transition-temperature superconductor and metallic phases. A fourth phase, ofunknown identity, occurs at light doping along the zero-temperature bound of the `pseudogap' regime. This regime is rich in peculiar electronic phenomena, prompting numerous proposals that it contains some form of hidden electronic order. Here we present low-temperature electronic structure imaging studies of a lightly hole-doped copper oxide: Ca_{2-x}Na_{x}CuO_{2}Cl_{2}. Tunnelling spectroscopy (at energies |E|>100 meV) reveals electron extraction probabilities greatly exceeding those for injection, as anticipated for a doped Mott insulator. However, for |E|<100 meV, the spectrum exhibits a V-shaped energy gap centred on E=0. States within this gap undergo intense spatial modulations, with the spatial correlations of a four CuO_{2}-unit-cell square checkerboard, independent of energy. Intricate atomic-scale electronic structure variations also exist within the checkerboard. These data are consistent with an unanticipated crystalline electronic state, possibly the hidden electronic order, existing in the zero-temperature pseudogap regime of Ca_{2-x}Na_{x}CuO_{2}Cl_{2}.

cond-mat.supr-con↗

arXiv2004

Charge dynamics of Ca_{2-x}Na_{x}CuO_{2}Cl_{2} as a correlated electron system with the ideal tetragonal lattice

We report the reflectivity and the resistivity measurement of Ca_{2-x}Na_{x}CuO_{2}Cl_{2} (CNCOC), which has a single-CuO2-plane lattice with no orthorhombic distortion. The doping dependence of the in-plane optical conductivity spectra for CNCOC is qualitatively the same to those of other cuprates, but a slight difference between CNCOC and LSCO, i.e., the absence of the 1.5 eV peak in CNCOC, can be attributed to the smaller charge-stripe instability in CNCOC. The temperature dependence of the optical onductivity spectra of CNCOC has been analyzed both by the two-component model (Drude+Lorentzian) and by the one-component model (extended-Drude analysis). The latter analysis gives a universal trend of the scattering rate Gamma(omega) with doping. It was also found that Gamma(omega) shows a saturation behavior at high frequencies, whose origin is the same as that of resistivity saturation at high temperatures.

cond-mat.supr-con↗

arXiv2004

Missing Quasiparticles and the Chemical Potential Puzzle in the Doping Evolution of the Cuprate Superconductors

The evolution of Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ from Mott insulator to superconductor was studied using angle-resolved photoemission spectroscopy. By measuring both the excitations near the Fermi energy as well as non-bonding states, we tracked the doping dependence of the electronic structure and the chemical potential with unprecedented precision. Our work reveals failures in the conventional quasiparticle theory, including the broad lineshapes of the insulator and the apparently paradoxical shift of the chemical potential within the Mott gap. To resolve this, we develop a model where the quasiparticle is vanishingly small at half filling and grows upon doping, allowing us to unify properties such as the dispersion and Fermi wavevector with the behavior of the chemical potential.

cond-mat.supr-con↗

arXiv2004

Imaging nano-scale electronic inhomogeneity in lightly doped Mott insulator Ca_{2-x}Na_{x}CuO_{2}Cl_{2}

The spatial variation of electronic states was imaged in the lightly doped Mott insulator Ca_{2-x}Na_{x}CuO_{2}Cl_{2} using scanning tunneling microscopy / spectroscopy (STM/STS). We observed nano-scale domains with a high local density of states within an insulating background. The observed domains have a characteristic length scale of 2 nm (~4-5a, a:lattice constant) with preferred orientations along the tetragonal [100] direction. We argue that such spatially inhomogeneous electronic states are inherent to slightly doped Mott insulators and play an important role for the insulator to metal transition.

cond-mat.supr-con↗

arXiv2004

Momentum anisotropy of the scattering rate in cuprate superconductors

We examine the momentum and energy dependence of the scattering rate of the high temperature cuprate superconductors using angle resolved photoemission spectroscopy. The scattering rate is of the form a + b*w. The inelastic coefficient b is found to be isotropic. The elastic term, a, however, is found to be highly anisotropic in the pseudogap phase of optimal doped samples, with an anisotropy which correlates with that of the pseudogap. This can be contrasted with heavily overdoped samples, which show an isotropic scattering rate in the normal state.

cond-mat.str-el↗